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Nitrogen oxides (NOx) have fallen steadily across the US over thelast 15 years. At the same time, NOx concentrations decrease onweekends relative to weekdays, largely without co-occurring changes in othergas-phase emissions, due to patterns of diesel truck activities. These trendstaken together provide two independent constraints on the role of NOxin the nonlinear chemistry of atmospheric oxidation. In this context, weinterpret interannual trends in wintertime ammonium nitrate(NH4NO3) in the San Joaquin Valley of California, a location withthe worst aerosol pollution in the US and where a large portion of aerosolmass is NH4NO3. Here, we show that NOx reductions havesimultaneously decreased nighttime and increased daytime NH4NO3production over the last decade. We find a substantial decrease inNH4NO3 since 2000 and conclude that this decrease is due toreduced nitrate radical-initiated production at night in residual layersthat are decoupled from fresh emissions at the surface. Further reductionsin NOx are imminent in California, and nationwide, and we make aquantitative prediction of the response of NH4NO3. We show thatthe combination of rapid chemical production and efficient NH4NO3loss via deposition of gas-phase nitric acid implies that high aerosol days incities in the San Joaquin Valley air basin are responsive to local changesin NOx within those individual cities. Our calculations indicate thatlarge decreases in NOx in the future will not only lower wintertimeNH4NO3 concentrations but also cause a transition in thedominant NH4NO3 source from nighttime to daytime chemistry

On the effectiveness of nitrogen oxide reductions as a control over ammonium nitrate aerosol